If you have only one IP, the answer from the_spy is correct- If you have more then one IP, cou can use port 443 for the ISPConfig webserver and on the main webserver by restricting the ISPConfig httpd to listen only to one IP address and restrict the main webserver to listen to all other IP addresses except the IP where the ISPConfig server listens on.
It is obvious that port 81 is not the best. Does it work in the same way with other products similar to ISPConfig?
Generally port 81 works fine, though it might not work in your case. Some panels use e.g. port 81 or another port or they use different IP addresses as you can do it in ISPConfig as well.